1. Field of the Invention
The present invention relates to a multiplexer or demultiplexer optical component and, in particular, to a multiplexer or demultiplexer for a wavelength division multiplexed optical system.
2. Discussion of Related Art
Wavelength division multiplexing has become a standard in optical networks over the last few decades. Wavelength division multiplexing (WDM) exploits the potential bandwidth of optical fibers by transmitting data over several channels on the same fiber. Each channel is transmitted on the optical fiber at a different wavelength. The rate of data transmission over the fiber, then, can be increased by a factor of M, where M is the number of channels (i.e., the number of different wavelengths) being transmitted over the fiber.
Recently, an explosion of WDM technologies has appeared on the market. Systems having 8, 16, and 32 channels have become commonplace. Dense WDM, DWDM, for example, has 32 channels following an ITU grid with 0.8 nm wavelength separation. However, in order to effectively utilize the bandwidth of the optical fiber, optical signals must be multiplexed and demultiplexed onto the fiber.
In WDM systems, optical signals are transmitted over a set of M channels. The M channels are multiplexed at the transmitter so that M wavelengths of light are simultaneously transmitted on an optical fiber to a receiver system. At the receiver system, the M channels are demultiplexed into optical signals transmitted at individual wavelengths of light. The individual wavelengths of light can then be directed to photodetectors so that the optical signals can be converted into electrical signals for processing by subsequent electronic circuitry.
In some demultiplexing systems, an optical fiber can be directly attached to a dielectric waveguide. The waveguide geometry exploits interference and/or diffraction in order to separate different wavelength constituents of the input light beam. These systems are difficult to fabricate, have large insertion losses, and are only applicable to single-mode fibers.
Demultiplexing can be accomplished with diffraction gratings, prisms, or filters, for example. The major problem with such devices is that they often include bulky and costly lenses and such which are very hard to reliably align, leading to large manufacturing costs and a bulky final product.
FIG. 1A, for example, shows an embodiment of a filter-based demultiplexer as described in European Patent Application EP 1 004 907 A2 by Lemoff et al. The demultiplexer of FIG. 1A includes a main optical block 14, an input surface 38, an objective mirror 40, wavelength-specific dielectric interference filters 20, 22, 24 and 26 coupled to main optical block 14, and a series of relay mirrors 30, 32, and 36 integrated into main optical block 14 to direct and focus light onto filters 20, 22, 24, and 26. As can be seen, when properly aligned light from fiber 42 is reflected from objected mirror 40 onto filter 20. Light not passed by filter 20 is reflected to mirror 30, which focuses and reflects light onto filter 22. Light not passed by filter 22 is reflected to mirror 32, which focuses and reflects light onto filter 24. Light not passed by filter 24 is reflected to mirror 36, which focuses and reflects light onto filter 26. Light that passes through filters 20, 22, 24, and 26 is focused onto detectors 60, 62, 64, and 66, respectively, by lenses 50, 52, 54, and 56, respectively. However, the difficulty in assembling and aligning demultiplexer 10 is great, which increases the manufacturing cost because of time spent in active alignment of components. Additionally, light is not incident on detectors 60, 62, 64 and 66 normal to the detection surfaces, causing the lens assembly to be less tolerant to lateral misalignment of the detector. Finally, light is not incident on lenses 50, 52, 54, and 56 parallel to the optical axis of lenses 50, 52, 54, and 56 causing greater light spread through aberration and ultimately leading to a system that is less tolerant to lens misalignment.
FIG. 1B shows an embodiment of a demultiplexer as described in U.S. Pat. No. 5,894,535, issued on Apr. 13, 1999, to Lemoff et al. Light from filter 140 is coupled into waveguide 122 formed on substrate 121. A trench 125 is formed in substrate 121 and filters 127a through 127d are placed into trench 125 to intersect light transmitted by waveguide 122. Light transmitted through filters 127a through 127d are coupled into waveguides 126a through 126d, respectively. Light reflected from filters 127a through 127c is reflected back to filters 127b through 127d, respectively, by mirror 123. From waveguides 126a through 126d, light can be coupled to optical detectors or optical fiber. The demultiplexer shown in FIG. 1B requires processing of a silicon wafer to form waveguides 122, waveguides 126a through 126d, and trench 125. It is difficult to design the waveguide for both a multimode and single mode fiber input in such a way that the insertion losses in the waveguide are similar, due to the different input profiles that the waveguide has to accept in the case of multimode and single mode fiber. Additionally, aligning filters 127a through 127d with waveguides 122 and 126a through 126d and subsequent alignment of optical detectors with waveguides 126a through 126d would be difficult and time consuming, thus increasing the cost of production.
FIG. 1C shows yet another optical demultiplexer. In the optical demultiplexer of FIG. 1C, light from optical fiber 150 is transmitted, through lens 151, to filter 152. Filters 152, 158, 164, 155, and 161 transmit light in particular narrow bands and reflect light outside of that band. Light transmitted through filters 152, 158, 164, 155, and 161 are transmitted through lenses 153, 159, 165, 156, and 162, respectively, to optical detectors 154, 160, 166, 157, and 163, respectively. Light reflected from filters 152, 155, 158, and 161 are incident on filters 155, 158, 161, and 164, respectively. The optical demultiplexer of FIG. 1C requires significant time and effort to align, significantly increasing the cost of production of the optical device.
Therefore, there is a need for optical multiplexer and demultiplexer devices for WDM optical systems that are easily aligned and assembled and which result in low insertion loss.